Early Cretaceous I‐type granites in the southwest Fujian Province: new constraints on the late Mesozoic tectonic evolution of southeast China

Zircons from two samples of the Sukeng pluton in the southwest Fujian Province, China, were analyzed by LA–ICP–MS with the aim of determining the timing of formation. The zircons from the two samples yield similar U–Pb ages of 100.47 ± 0.42 and 102.46 ± 0.69 Ma, indicating that the Sufeng pluton is contemporaneous with the Sifang and Luoboling plutons, all of which are also related to Cu–Au–Pb–Zn–Mo mineralization within the study area. All three plutons have geochemical features of I‐type granites, are high‐ to mid‐K calc‐alkaline metaluminous rocks, and have average molar Al₂O₃/ (CaO+Na₂O+K₂O) values of 0.95, initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70465–0.70841, εNd(t) values at 101 Ma from –1.72 to –7.26, and two‐stage Nd model ages (T_2DM) from 1.16 to 1.60 Ga. Zircons within these plutons have εHf(t) values at 101 Ma from –3.5 to 6.25 and T_2DM ages from 0.74 to 1.46 Ga, suggesting these I‐type granites formed from magmas generated by partial melting of Mesoproterozoic to Neoproterozoic continental crust that mixed with mantle‐derived magmas. The magmatism was associated with thickening of the lower crust caused by collisions between microcontinents in the Cathaysian Block, which were driven by Early Cretaceous subduction of the Pacific Plate.     

华南晋宁期造山运动──地质年代学和地球化学制约

综合了前人和作者最新的地质年代学和地球化学研究成果,对华南晚元古代晋宁期造山运动的演化提供了化学地球动力学制约晚元古代早期（0．97－1．0Ga）的皖南、赣东北和桂北蛇绿岩在元素地球化学上具有相似的岛弧特征,但它们的Nd同位素组成明显不同新获得的高精度结石U－Ph年龄资料表明,桂北地区的本洞、三防和元宝山等三个主要花岗岩岩体均形成于820-825Ma,排除了本区有中元古代晚期花岗岩的存在．扬子南缘中元古代到三叠纪各个时代地层中的泥质沉积岩的TDM在晚元古代早期从约1．8Ga急剧降低至约1．3Ga,反映了这个时期的沉积物源区有大量新生馒源物质的加入．根据研究资料,提出了华南和扬子块体晋宁期陆-弧-陆碰撞模式     

The petrogenesis and implications of deformed late Caledonian–early Hercynian granites in the Wuyi area,South China

Precambrian basement rocks have been affected by Caledonian thermal metamorphism. Caledonian‐aged zircon grains from Precambrian basement rocks may have resulted from thermal metamorphism. However, Hercynian ages are rarely recorded. Zircon U–Pb Sensitive High Resolution Ion Microprobe (SHRIMP) dating reveals that zircon ages from the Huyan, Lingdou, and Pengkou granitic plutons can be divided into two groups: one group with ages of 398.9 ±5.3 Ma, 399 ±5 Ma, and 410.2 ±5.4 Ma; and a second group with ages of 354 ±11 Ma, 364.6 ±6.7 Ma, and 368 ±14 Ma. The group of zircon U–Pb ages dated at 410–400 Ma represent Caledonian magmatism, whereas the 368–354 Ma ages represent the age of deformation, which produced gneissosity. The three plutons share geochemical characteristics with S‐type granites and belong to the high‐K calc‐alkaline series of peraluminous rocks. They have (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.710 45–0.724 68 and εNd(t) values of ?7.33 to ?10.74, with two‐stage Nd model ages (T_DM2) ranging from 1.84 Ga to 2.10 Ga. Magmatic zircon εHf(t) values range from ?3.79 to ?8.44, and have T_DMC ages of 1.65–1.93 Ga. The data suggest that these granites formed by partial melting of Paleoproterozoic to Mesoproterozoic continental crust. A collision occurred between the Wuyi and Minyue microcontinents within the Cathaysia Block and formed S‐type granite in the southwest Fujian province. The ca 360 Ma zircon U–Pb ages can represent a newly recognized period of deformation which coincided with the formation of the unified Cathaysia Block.     

Hafnium isotopes in Jack Hills zircons and the formation of the Hadean crust

New bulk Hf and Pb isotope data were obtained for 63 leached single zircons from Jack Hills (JH), Western Australia, using solution chemistry and, respectively, MC-ICP MS and ICP-MS. With the exception of one “young” zircon at 3.32 Ga, the remainder of the selected grains were previously dated at > 3.9 Ga by ion-microprobe. This work extends and complements the solution chemistry data of Harrison et al. [Harrison, T.M., Blichert-Toft, J., Müller, W., Albarède, F., Holden, P., Mojzsis, S.J., 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science 310, 1947–1950.] but uses bulk rather than in situ Pb–Pb ages to interpret the Hf isotope data. This larger data set is used to explore whether the host rocks of the JH zircons formed as a succession of pulses or rather as a single event, and to calculate the age and assess the nature of their crustal protolith. We find that the parent granites of the JH zircons analyzed here formed during a single pulse 4.1 ± 0.1 Ga ago by the remelting of a 4.30–4.36 Ga old protolith. Monte Carlo modeling indicates that the ¹⁷⁶Lu/¹⁷⁷Hf ratios of this material (< 0.01) are unlike the ratios of modern-type oceanic crust and island arc rocks but rather fit a tonalite–trondhjemite–granodiorite (TTG) source. TTGs themselves derived their inordinately enriched character from a basaltic progenitor which corresponds to the missing enriched reservoir identified by the ¹⁴³Nd–¹⁴⁴Nd, ¹⁴²Nd–¹⁴⁴Nd, and ¹⁷⁶Hf/¹⁷⁷Hf systematics of Archean rocks. We speculate that crystallization of the magma ocean in the presence of garnet left the upper mantle and an early basaltic crust enriched in incompatible elements. Reaction of this early crust with the overlying hydrosphere and subsequent foundering into the mantle gave rise at ～ 4.3 Ga to the TTG protolith of the JH granites. Dating the onset of plate tectonics therefore depends on whether TTGs can be considered as subduction zone magmas or not.     

湘西南兰蓉岩体锆石SHRIMP U-Pb年龄、地球化学特征及构造背景

湘西南兰蓉岩体为一加里东期小侵入体,由黑云母二长花岗岩和二云母二长花岗岩组成.（443.5±8.1）Ma的锆石SHRIMP U Pb年龄表明花岗岩形成于早志留世早期.主量元素组成表明岩体总体属钙碱性高钾钙碱性系列强过铝质花岗岩类.该侵入体Ba、（Ta＋Nb）、Sr、P、Ti强烈亏损,Rb、（Th＋U＋K）、（La＋Ce）、Nd、（Zr＋Hf＋Sm）、（Y＋Yb＋Lu）等相对富集;稀土元素含量较高、轻稀土富集明显、Eu显著亏损;Isr值为0.71299,εSr（t）值为120,εNd （t）值为 8.11和-8.89,t2DM为1.82和1.84Ga.C/MF-A/MF图解显示其源岩为泥质岩和砂屑岩.上述地球化学特征表明兰蓉岩体为陆壳碎屑岩石部分熔融形成的S型花岗岩.基于岩石成因、构造环境判别以及区域构造演化过程,推断兰蓉岩体的具体形成机制为：奥陶纪末志留纪初的北流运动（板内造山运动）导致地壳增厚、升温,尔后在挤压减弱、应力松弛的后碰撞减压构造环境下,中、上地壳酸性岩石发生部分熔融并向上侵位而形成兰蓉岩体.     

SHRIMP zircon and EPMA monazite dating of granitic rocks from the Maizuru terrane, southwest Japan: Correlation with East Asian Paleozoic terranes and geological implications

Abstract The Maizuru terrane, distributed in the Inner Zone of southwest Japan, is divided into three subzones (Northern, Central and Southern), each with distinct lithological associations. In clear contrast with the Southern zone consisting of the Yakuno ophiolite, the Northern zone is subdivided into the western and eastern bodies by a high-angle fault, recognized mainly by the presence of deformed granitic rocks and pelitic gneiss. This association suggests an affinity with a mature continental block; this is supported by the mode of occurrence, and petrological and isotopic data. Newly obtained sensitive high mass-resolution ion microprobe (SHRIMP) zircon U–Pb ages reveal the intrusion ages of 424 ± 16 and 405 ± 18 Ma (Siluro–Devonian) for the granites from the western body, and 249 ± 10 and 243 ± 19 Ma (Permo–Triassic) for the granodiorites from the eastern body. The granites in the western body also show inherited zircon ages of around 580 and 765 Ma. In addition, electron probe microanalysis (EPMA) monazite U–Th–total Pb dating gives around 475–460 Ma. The age of intrusion, inherited ages, mode of occurrence, and geological setting of the Siluro–Devonian granites of the Northern zone all show similarities with those of the Khanka Massif, southern Primoye, Russia, and the Hikami granitic rocks of the South Kitakami terrane, Northeast Japan. We propose that both the Siluro–Devonian and Permo–Triassic granitic rocks of the Northern zone are likely to have been juxtaposed through the Triassic–Late Jurassic dextral strike-slip movement, and to have originated from the Khanka Massif and the Hida terrane, respectively. This study strongly supports the importance of the strike-slip movement as a mechanism causing the structural rearrangement of the Paleozoic–Mesozoic terranes in the Japanese Islands, as well as in East Asia.     

Newly discovered Late Cretaceous adakites in South Fujian Province: Implications for the late Mesozoic tectonic evolution of Southeast China

The Yongchun pluton is a Late Cretaceous adakitic intrusion in South Fujian Province, Southeast China, with associated metal mineralization. An understanding of the Yongchun pluton is helpful in tectono‐magmatic evolutionary processes, and is important in explaining the origin of related porphyry‐type deposits. Zircons from three samples of the pluton were analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS), yielding U–Pb ages of 99.50 ±0.87 Ma, 97.74 ±0.59 Ma, and 99.65 ±0.92 Ma. These ages are similar to those of the Sifang, Luoboling, and Sukeng plutons, all of which are related to Cu–Pb–Zn–Mo mineralization within the study area. The Yongchun pluton comprises high‐potassium, calc‐alkaline, metaluminous rocks, with average A/CNK values of 0.91, ⁸⁷Sr/⁸⁶Sr ratios of 0.705 51 to 0.706 83, εNd(t) values of ?4.63 to ?5.90, and two‐stage Nd model (T_2DM) ages of 1.49–1.39 Ga, indicating the magmas were generated by partial melting of Mesoproterozoic continental crust mixed with mantle‐derived magmas. The pluton has geochemical characteristics typical of adakites, such as a high Sr content (average 553 ppm), and low Y (average 15.2 ppm) and Yb (average 1.61 ppm) contents, indicating that the parental magma was formed under high‐pressure conditions. The magmatism was associated with thickening of the lower crust during a change in subduction angle and convergence rate of the paleo‐Pacific Plate at 100 Ma. The compression was limited to South Fujian Province.     

Development of Archaean lithosphere deduced from chronology and isotope chemistry of Scourie Dykes

The chronology and isotope geochemistry of a selection of Proterozoic Scourie dykes has been investigated in order to specify both their time of emplacement within the thermal history of the Archaean crust of N.W. Scotland, and to attempt to characterise the evolution of continental lithosphere. SmNd, RbSr and UPb isotope analyses are presented. Primary, major igneous minerals separated from four well preserved dykes yield SmNd ages of 2.031 ± 0.062Ga, 2.015 ± 0.042Ga, 1.982 ± 0.044Ga and 2.101 ± 0.078Ga, which are interpreted as crystallisation ages.The initial Nd isotope compositions in the dykes at their emplacement age of 2.0 Ga, range from +3.4 to −6.8, indicating the presence of an older lithospheric component. SmNd whole-rock isotope data for fifteen dykes, if interpreted to have age significance, yield an “age” of 3.05 ± 0.27 Ga. SmNd crustal residence ages for the same dykes average 2.95 Ga, which is interpreted as the time that small melts were added to the Lewisian lithosphere. The possibility that correlated¹⁴⁷Sm/¹⁴⁴Nd and¹⁴³Nd/¹⁴⁴Nd ratios are an artifact of mixing between depleted mantle melts generated at 2.0 Ga, and an older enriched lithospheric component is not eliminated by the data, but the relationship between 1/Nd and¹⁴³Nd/¹⁴⁴Nd ratios rules out any simple mixing. UPb isotope data for plagioclase feldspars and whole-rock samples of dykes provide useful estimates of initial Pb-isotope composition of the dykes at the time of their emplacement. Initial²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb ratios vary considerably and range from 13.98 to 15.78, and 14.72 to 15.56 respectively, and suggest that the UPb fractionation responsible must have occurred at least 2.5 Ga ago.The Scourie dykes have inherited a trace element enriched component from the Lewisian lithosphere, which has resided there since ca. 3 Ga ago. Whether the dykes inherited this material from the crust or the mantle portions of the lithosphere or both, it seems likely that small basaltic melts derived from asthenospheric mantle were ultimately responsible for the enrichment. The simplest view is that these small melt fractions had been resident in the mantle part of the Lewisian lithosphere. In this case the Archaean trace-element enrichment and element fractionation in the Lewisian lithospheric mantle sampled by the dykes was closely associated in time with the generation of the 2.9 Ga old crustal portion of the lithosphere [36,37].     

Absence of Archean basement in the South Kunlun Block: Nd-Sr-O isotopic evidence from granitoids

Abstract   The West Kunlun mountain range along the northwestern margin of the Tibetan Plateau is crucial in understanding the early tectonic history of the region. It can be divided into the North and South Kunlun Blocks, of which the former is considered to be part of the Tarim Craton, whereas consensus was not reached on the nature and origin of the South Kunlun Block. Samples were collected from the 471 Ma Yirba Pluton, the 405 Ma North Kudi Pluton and the 214 Ma Arkarz Shan Intrusive Complex. These granitoids cover approximately 60% of the Kudi area in the South Kunlun Block. Sr, Nd, and O isotope compositions preclude significant involvement of mantle-derived magma in the genesis of these granitoids; therefore, they can be used to decipher the nature of lower–mid crust in the area. All samples give Mesoproterozoic Nd model ages (1.1–1.5 Ga) similar to those of the exposed metamorphic complex of this block but significantly different from those of the basement of the North Kunlun Block (2.8 Ga). This indicates that the South Kunlun Block does not have an Archean basement, and, thus, does not support the microcontinent model that suggests the South Kunlun Block was a microcontinent once separated from and later collided back with the North Kunlun Block.     

南海瑞雷面波群速度层析成像及其地球动力学意义

南海处于欧亚板块、 菲律宾海板块、 太平洋板块和印度-澳大利亚板块的交汇处, 其地质和构造作用十分复杂．通过面波群速度成像, 给出了南海及邻区的三维横波速度分布并分析了其地球动力学意义．南海西部和南部新布设的地震台站使得利用单台法时路径覆盖比过去更好. 特别是在华南地区, 新的台站分布能够弥补该地区地震少且台站少造成的射线密度不够的缺点． 首先运用多重滤波法得到南海周边48个台站周期为14——130 s范围内的基阶瑞雷波频散曲线图; 接着通过子空间反演得到整个区域在不同周期时的群速度分布; 最后通过阻尼最小二乘反演得到不同深度切片上的横波速度分布及不同纵剖面上的横波速度分布． 结果显示: ① 海盆速度较高, 且速度分布很好地勾勒出海盆的轮廓. 浅层较高的横波速度说明海盆都具有洋壳性质, 而深部较高的横波速度则可能对应扩张中心生成洋壳后残留的高速物质． 不同海盆速度上的差异与它们的热流值和年龄大小一致．海盆下的高速异常在60 km以下消失, 且在一定深度范围内由低速区替代． 在低速区下200 km深度, 在南海海盆观测到一条NE-SW走向的高速异常, 可能与古俯冲带有关. ② 环南海出现明显的高速区, 对应俯冲带特征, 且这些高速区速度差异明显且有间断, 说明俯冲带的非均质性和俯冲角度的差异. ③ 在环南海高速区内侧（向南海侧）观测到不连续的低速区． 在浅层, 这些低速区反映了沉积层和地壳的厚度特征. 在地幔, 这些低速区可能对应于古太平洋俯冲带的地幔楔或者也可能反映了南海海盆停止扩张后残留的地幔熔融物质. ④ 南海海盆岩石圈的厚度为60——85 km．      

Geochronology and Nd isotopic data of Grenville-age rocks in the Colombian Andes: new constraints for Late Proterozoic-Early Paleozoic paleocontinental reconstructions of the Americas

New UPb zircon crystallization ages and ⁴⁰Ar/³⁹Ar cooling ages from the Colombian Andes confirm the existence of rocks metamorphosed during the Orinoquian Orogenic Event (ca. 1.0 Ga) of northern South America. ε_Nd (t = 1.1 Ga) for these rocks range from −3.9 to +0.91, which is interpreted as a mixture of Late Archean-Early Proterozoic crust with juvenile material produced during the 1.1 Ga orogenic event. The Colombian Grenville age rocks are part of a much longer metamorphic pericratonal belt, sporadically exposed along the Andes, in western-central Peru, southern Bolivia and northern Argentina. In addition, Nd model (T_DM) ages for the Colombian rocks range from 1.9 to 1.45 Ga, similar to those obtained in the Grenville Province of the eastern U.S. and in the Mexican basement, placing constraints on Late Proterozoic-Early Paleozoic paleocontinental reconstructions.     

A magnetotelluric study of the Damara Belt in Namibia: 1. Regional scale conductivity anomalies

The Namibian margin is dominated by the late Proterozoic to early Cambrian fold belts of the Damara Orogen, which wrap around and separate the Congo and Kalahari Cratons. This mosaic of relatively ‘soft’ fold belts and ‘hard’ cratons apparently controlled the path for the opening of the South Atlantic in the early Cretaceous. The continents split along the coast-parallel fold belts of the Damara Orogen while the inland fold belt (Damara Belt) was effected by extension and widespread igneous intrusion but never developed to the rift stage. This paper is concerned with the interpretation of magnetotelluric (MT) data along a 200 km NW-SE profile across the Damara Belt in NW Namibia. The regional, two-dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones: (i) a generally very resistive upper crust which is typical for the granites and metasediments of the Damara Belt, (ii) two subvertical conductors in upper to mid-crustal levels which correlate with major tectonic zone boundaries and (iii) a highly conductive middle to lower crust in the southern part of the profile. The geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust. We interpret the high electrical conductivity in terms of graphite (or other forms of mineralization) enrichment along the shear planes. This zone of crustal weakness may have originated in Pre-Damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the Cretaceous rifting and magmatism associated with the opening of the South Atlantic.     

Sr–Nd isotope ratios of gabbroic and dioritic rocks in a Cretaceous-Paleogene granite terrain, Southwest Japan

Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial ⁸⁷Sr/⁸⁶Sr (0.7063–0.7076) and low initial Nd isotope ratios (?_Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial ⁸⁷Sr/⁸⁶Sr (usually less than 0.7060) and higher Nd isotope ratios (?_Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.     

马尼拉俯冲带缺失中深源地震成因初探

马尼拉俯冲带是整个南海地震活动多发区,地震成因与南海的形成和构造演化关系密切.对马尼拉俯冲带地震数据和层析成像结果进行了深入分析.结果表明:马尼拉俯冲带的地震活动主要为密集的浅源地震,缺失中深源地震.进一步分析揭示:①脱水和榴辉岩的形成在南海洋壳到达软流圈前就基本停止.马尼拉俯冲带南部在较浅的深度就转变为塑性变形,并停...     

南海北部洋陆转换带盆地发育动力学机制

南海北部洋陆转换带是近年来基础科学研究和深水油气勘探热点地区.本文在详细研究南海北部洋陆转换带新采集的二维长电缆深反射地震剖面资料的基础上,采用挠曲悬臂梁模型和挠曲回剥模型算法,分别计算了上地壳、地壳和整个岩石圈拉伸系数,实验结果表明,研究区洋陆转换带盆地岩石圈发生了与深度相关的拉伸变形过程,并且随深度增加,拉伸量逐渐变大,该结果解释了南海北部盆地裂后阶段发生的加速沉降现象.同时,本文结合南海北部洋陆转换带盆地发育过程的特点,将洋陆转换带盆地演化划分为陆内裂陷阶段、裂后热沉降阶段和裂后加速沉降阶段.本研究将有助于认识南海北部深水盆地特征,并对大陆边缘动力学研究和陆缘盆地深水区油气勘探有重要意义.     

Geochemical and geochronological studies of the Muong Hum alkaline granitic pluton from the Phan Si Pan Zone,northwest Vietnam: Implications for petrogenesis and tectonic setting

The Phan Si Pan zone in northwest Vietnam is an important tectonic unit for understanding the geological evolution of the southeast Asian Block. Numerous late Permian A‐type granites outcrop in this zone. In this study, new geochemical and geochronological data derived from the Muong Hum alkaline granitic pluton in the Phan Si Pan zone were investigated for its petrogenesis and tectonic setting. Zircon U–Pb analyses of three samples yielded ²⁰⁶Pb/²³⁸U ages of (251.1 ±3.5) Ma, (251.2 ±3.8) Ma, and (253.9 ±1.5) Ma, respectively, coinciding with the ages of the acid member of magma from Emeishan large igneous province, southwest China. The Muong Hum granite has 10 000 × Ga/Al and A/CNK values of 4.70–4.93 and 0.87–0.90, respectively, as well as negative Eu anomalies. It shows significant depletion of Ba, Sr, Ti, and P, similar to features of A‐type granite. Zircons have positive ε_Hf(t) values (+1.9 to +8.6) and Hf model ages (T_DM1) of 595–846 Ma, originating a mantle source. Compared with the Panzhihua A‐type granite of the southwest China domain and other A‐type plutons of the Phan Si Pan zone, including Ye Yen Sun, Phu Sa Phin, Nam Xe, Tam Duong Phan Si Pan, and Taihe, the geochemical characteristics and zircon Hf isotopic compositions of the Muong Hum granite demonstrate an affinity of mantle magma. It is believed that the Phan Si Pan zone is an important part of Emeishan large igneous province. It was reworked by the Cenozoic Aillaoshan‐Red River shear fault to its present location.     

川滇交界东段昭通、莲峰断裂带的深部结构特征与2014年鲁甸M_S6.5地震

2014年鲁甸M_S6.5地震位于川滇菱形块体向东突出的过渡变形区大凉山次级块体南东缘的昭通、莲峰断裂带内部,属于青藏高原东南缘南北地震带的中南段,近十多年来,该断裂带及其周边中强地震的发生频次明显增多,昭通、莲峰断裂带是否具备孕育和发生强震的深部构造背景成为一个亟待研究的问题.为了研究昭通、莲峰断裂带的深部结构特征及孕震背景,探求2014年鲁甸M_S6.5地震的成因的深部动力机制,本文充分收集了四川、云南等区域数字地震台网和"中国地震科学台阵探测-南北地震带南段"("喜马拉雅"项目Ⅰ期)流动地震台阵的观测数据,应用区域震和远震联合反演的方法得到川滇地区三维速度结构图像,在此基础上重点剖析和研究了昭通、莲峰断裂带P波速度结构;再对昭通、莲峰断裂带及周边区域的重力、航磁数据进行三维视密度和视磁化强度反演,得到了壳内不同深度层视密度的横向变化特征和反映壳内磁性物质的分布范围以及结晶基底的视磁化强度异常分布情况,综合分析研究昭通、莲峰断裂带的深部结构特征及孕震动力环境.研究结果表明:川滇交界东部昭通、莲峰断裂带及其周边地区上地壳物质存在显著的横向介质差异,中下地壳深度范围大凉山次级块体西南缘存在低速异常分布,并呈现出近SN向的展布特征,2014年鲁甸M_S6.5地震位于该高低速异常的分界线附近略偏向高速体一侧.P波速度结构还揭示了鲁甸M_S6.5主震震源体下方中下地壳存在大范围低速异常分布,P波速度异常扰动与重磁异常的展布特征、梯度变化在深度和分区特征上均具有较好的联系和可比性,结合昭通、莲峰断裂带中下地壳范围内存在大范围的低密度弱磁性异常分布,综合表明了该区中下地壳物质相对较为软弱,这种特有的深部物性结构特征有利于应力在脆性的上地壳内积累和集中.研究结果还揭示了共轭断裂的深部构造形态,高低航磁异常边界与NW向的苞谷脑—小河断裂的深部展布形态相一致,苞谷脑—小河断裂处于航磁异常突变带附近,昭通断裂北段(昭通—鲁甸段)位于上地壳强磁性、高波速异常区内且具有深大断裂的深部地球物理场响应特征,因此该断裂段(昭通—鲁甸段)具备发生7级及以上强震的深部构造背景.当大凉山次级块体内部的中下地壳低速管流层自NW向SE方向运动到昭通、莲峰断裂带附近时,受到华南块体的强烈阻挡,应力在昭通、莲峰断裂附近基底性质存在差异处集中,脆性上地壳中低强度区域在横向挤压的构造应力场作用下易于破裂从而引发强震,这也正是昭通、莲峰断裂带内部鲁甸M_S6.5地震孕育和发生的深部构造环境.     

青藏高原震源分布与板块运动

本文分析了青藏高原及其邻区大量近期地震的震源深度分布资料,发现中源地震不仅分布在众所周知的兴都库什和印缅山弧一带,而且在印度洋板块与欧亚板块汇聚带印度河-雅鲁藏布江以南,以及欧亚板块内部的帕米尔、西昆仑、柴达木和天山南缘一带也有中源地震分布,它们构成了这一地区三条向南倾斜的震源带。 这些中源地震震源带的存在表明,向北运动的印度次大陆与亚洲大陆碰撞以后,印度次大陆北缘本身并没有消减,而是迫使亚洲大陆通过三条向南倾斜的岩石层消减带产生了大规模的消减作用。 中源地震在平面上分布的不连续性,揭示了这一地区的许多条走滑断层的现代活动。这些走滑断层的巨大位移显示了青藏高原内部各块体之间的横向运动也是很可观的。 最后,提出了亚洲大陆多条南倾消减带的形成和发展模式。     

郯庐断裂带及其邻区上地幔顶部Pn波速度与各向异性层析成像

本研究拾取了中国数字测震台网固定台站记录的2008-2016年2级以上地震事件中的27233条高质量Pn到时资料,反演得到了郯庐断裂带及其邻区上地幔顶部Pn波速度和各向异性结构模型.结果显示,研究区上地幔顶部Pn波速度结构存在强烈的横向不均匀性,速度异常形态与区域地质构造较为吻合.太行山造山带、鲁西隆起、大别造山带、苏鲁褶皱带、胶辽隆起和华北盆地南端等隆起区表现为低波速异常,而黄海北、南部盆地、渤海湾和华北盆地北部等凹陷区均为高波速异常.壳内强震主要发生在Pn低波速异常和高低波速异常的横向过渡地带,说明强震的发生与上地幔结构的横向变化之间存在有一定关联.郯庐断裂带两侧Pn波速度以郯城地震为界其东北侧和西南侧分别分布有与断裂带近平行的低波速异常条带,而西北侧和东南侧分别分布有高波速异常条带,各向异性快波方向近乎沿断裂带走向,可能由于上地幔热物质沿郯庐断裂带上涌形成低速异常后断裂带发生左旋平移运动所致.华北盆地内上地幔顶部Pn波速度结构和各向异性的明显变化,反映华北克拉通破坏过程中经历了地幔热物质上涌、莫霍面隆升以及岩石圈拆沉等复杂构造变形.     

Recycling of the continental crust

In order to understand the evolution of the crust-mantle system, it is important to recognize the role played by the recycling of continental crust. Crustal recycling can be considered as two fundamentally distinct processes: 1) intracrustal recycling and 2) crust-mantle recycling. Intracrustal recycling is the turnover of crustal material by processes taking place wholly within the crust and includes most sedimentary recycling, isotopic resetting (metamorphism), intracrustal melting and assimilation. Crust-mantle recycling is the transfer of crustal material to the mantle with possible subsequent return to the crust. Intracrustal recycling is important in interpreting secular changes in sediment composition through time. It also explains differences found in crustal area-age patterns measured by different isotopic systems and may also play a role in modeling crustal growth curves based on Nd-model ages. Crustal-mantle recycling, for the most part, is a subduction process and may be considered on three levels. The first is recycling with only short periods of time in the mantle (<10 m.y.). This may be important in explaining the origin of island-arc and related igneous rocks; there is growing agreement that 1–3% recycled sediment is involved in their origin. Components of recycled crustal material, with long-term storage (up to 2.5 b.y.) in the mantle as distinct entities, has been suggested for the origin of ocean island and ultrapotassic volcanics but there is considerably less agreement on this interpretation. A third proposal calls for the return of crustal material to the mantle with efficient remixing in order to swamp the geochemical and isotopic signature of the recycled component by the mantle. This type of recycling is required for steady-state models of crustal evolution where the mass of the continents remains constant over geological time. It is unlikely if crust-mantle recycling has exceeded 0.75 km³/yr over the past 1–2 Ga.Good evidence exists that selective recycling is an important process. Sedimentary rocks preserved in different tectonic settings are apparently recycled at different rates, resulting in a bias in the sediment types preserved in the geologic record. Selective recycling has important implications for the interpretation of Nd model ages of old sedimentary rocks and in the analysis of accreted terranes. Although there is evidence that continental crust was formed prior to 3.8 Ga, the oldest preserved rocks do not exceed this age. It is likely that the intense meteorite bombardment, which affected the earth during the period 4.56–3.8 Ga, coupled with rapid mantle convection, which resulted from greater heat production, caused the destruction and probable recycling into the mantle of any early formed crust.Although crust-mantle recycling is seen as a viable process, it is concluded that crustal growth has exceeded crust-mantle recycling since at least 3.8 Ga. Intracrustal recycling has not been given adequate consideration in models of crustal growth based on isotopic data (particularly Nd model ages). It is concluded that crustal growth curves based on Nd model ages, while vastly superior to those based on K/Ar or Rb/Sr, tend to underestimate the volume of old crust, due to crust-mantle and/or intracrustal recycling.